Electron capture detectors (ECD) for gas chromatographs are known which have a radioactive ionization source in the form of .sup.63 Ni foil, as well as a collecting electrode and lines for feeding in and removing gas to be analyzed, e.g. in U.S. Pat. No. 4,063,156, which is referred to in its entirety.
The disadvantage of such a detector with a radioactive ionization source is that there is a risk of radioactive contamination. In this context, attempts were made to design ECDs with non-radioactive ionization sources in the reaction chamber, such as in U.S. Pat. No. 3,149,279 and USSR A.C.N. 375548, etc.
The known ECD most similar to the ECD proposed in this application consists of two partial chambers separated by a partition which is permeable for electrons. A non-radioactive ionization source with thermoemitter is attached in the first partial chamber, and the collecting electrode is contained in the second partial chamber, which is connected with lines for the feed in and removal of gas to be analyzed.
Such an ECD is known from the publication EP 0 115 495 A1 (PCT-WO84/00604).
In the known detector, the partition which is permeable for electrons is a perforated plate, through the holes of which the electrons are transferred from the first to the second partial chamber by means of a gas stream.
The disadvantage of the known detector is that contact between the gas to be analyzed and the thermoemitter cannot be completely excluded. Contact can be caused by an interruption in the gas stream which supplies the second chamber with electrons. Passage of analysis gas molecules through the perforated partition can occur, e.g. as a consequence of an increase in pressure in the second chamber to which the gas is fed if chromatography columns with a programmed gas stream are used. Furthermore, when using capillary chromatography columns, one usually tries to reduce the chamber volume to which the gas to be analyzed from the chromatography column is fed. This would lead to a reduction in the distance from the ionization source, which in turn allows diffusion of the analysis gas molecules, against the gas stream, into the first partial chamber where the electron source is located. These are possible causes for contamination of the active surface of the electron source by molecules of the gas to be analyzed. As a result of this, the background stream changes, which for its part magnifies the instability of the measurement and the errors when determining the concentrations.
The objective of this invention is to produce an ECD with a non-radioactive electron source which prevents all contact between analysis gas and the active surface of the electron source.